﴿ يَوْمَ تَرْجُفُ ٱلرَّاجِفَةُ ﴾

“[HENCE, think of] the Day when a violent convulsion will convulse [the world],”

the day when the Tremor quakes that is to say the first blast as a result of which everything will be shaken violently thus it the subject has been described in terms of the effect it produces